The present invention relates to a syringe for injecting fluids into or withdrawing them from a patient, and more particularly, concerns a syringe useful for taking samples of blood from the patient.
In the procedure of withdrawing blood from a patient for sampling purposes, the blood is collected in a syringe following which it is analyzed. The standard syringe includes a barrel into which a plunger is slidably fit and a needle projecting from the barrel. Following venipuncture of the patient, the blood travels through the hollow needle upon rearward withdrawal of the plunger. When sufficient volume of blood has been collected, the withdrawal of the plunger is stopped and the syringe is removed from the patient. In using this procedure, it has been found that the clotting time of blood is significantly reduced by incorporating a small amount of anticoagulant into the blood sample. Some problems arise, however, with the inclusion of this anticoagulant in the blood sample.
Specifically, the use of anticoagulants, such as heparin, may affect various characteristics of the blood sample such as the partial pressure of carbon dioxide and the pH. Since it is desirable to pre-package the syringe with the anticoagulant in the barrel previous to the collection in the blood sample, it is imperative to control the volume of the anticoagulant in order to minimize or eliminate its effect on the blood characteristics. Accordingly, the volume inside the barrel requires an effective measure of control so that the amounts of anticoagulant may be regulated and, in addition, the amount of collected blood can be regulated so that the ratio of collected blood to anticoagulant can be a known factor.
Although various means have been known in the art of syringes for controlling plunger movement relative to the barrel, such as the control features found in U.S. Pat. Nos. 2,607,343 and 2,216,354, the art has been deficient in providing a suitable syringe to control both the inward and the outward movement of the plunger to effectively control the volume inside the barrel. Therefore, it is toward the solution of this deficiency that the present invention is directed.